Xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is para-xylene, the principal feedstock for polyester, which continues to enjoy a high growth rate from large base demand. Ortho-xylene is used to produce phthalic anhydride, which supplies high-volume but relatively mature markets. Meta-xylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but is usually considered a less-desirable component of C8 aromatics.
Among the aromatic hydrocarbons, the overall importance of xylenes rivals that of benzene as a feedstock for industrial chemicals. Xylenes and benzene are produced from petroleum by reforming naphtha but not in sufficient volume to meet demand, thus conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Often toluene is de-alkylated to produce benzene or selectively disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered.
An aromatics complex flow scheme has been disclosed by Meyers in the HANDBOOK OF PETROLEUM REFINING PROCESSES, 2d. Edition in 1997 by McGraw-Hill, and is incorporated herein by reference.
Traditional aromatics complexes send toluene to a transalkylation zone to generate desirable xylene isomers via transalkylation of the toluene with A9+ components. A9+ components are present in both the reformate bottoms and the transalkylation effluent.
Methylation of toluene or benzene with oxygenates such as methanol has been proposed as a pathway to make xylene and to increase methyl to phenyl ratio in the aromatic complex to maximize xylene production. Toluene methylation operating in vapor phase has a poor feed, especially oxygenate, utilization, low aromatics conversion per pass and poor catalyst stability in a time span of hours, days and weeks, thus requiring frequent regeneration. Typically, toluene methylation is operating with selective para-xylene production objective, which requires operating under severe process conditions, namely high temperature where methanol decomposition to COx and H2 is significant, with a significant amount of diluents such as H2O and H2 and thus requires recycling a catalyst which is relatively difficult to prepare reproducibly. MFI zeolite has been the catalyst being used predominantly in this process.
Toluene methylation can greatly increase the para-xylene production of an aromatic complex. However, the toluene methylation catalyst cannot effectively process un-extracted toluene. If toluene methylation uses unextracted toluene, non-aromatics can build up in the toluene methylation loop to nearly 25% of the feed. This represents a loss of capacity from a state of the art toluene methylation unit. To avoid this, toluene is traditionally extracted using an aromatics extraction unit, but an aromatics extraction unit is an expensive unit to run.
Accordingly, it is desirable to provide improved methods and apparatuses for methylation of aromatic compounds such as toluene and benzene in an aromatics complex. Further, it is desirable to provide a cost-effective method and apparatus for toluene and/or benzene methylation which operates under mild condition, promotes high utilization of the feedstock and where higher than equilibrium para-xylene to xylene can be achieved without using dilution. Also, it is desirable to reduce the overall costs of operating and/or incorporating such a methylation unit in an aromatics complex. Furthermore, other desirable features and characteristics of the present subject matter will become apparent from the subsequent detailed description of the subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the subject matter.